Watch lume explained, and how to charge it
How the glow on a watch dial works, why it fades through the night, how to charge it, and the difference between Super-LumiNova, tritium and vintage radium.
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How it glows
Almost every modern watch uses a strontium aluminate pigment, sold as Super-LumiNova, LumiBrite or Chromalight. It soaks up light and gives it back slowly. It glows brightest straight after charging and fades over the next few hours, but stays readable in the dark much longer.
How to charge it
- A few minutes in sunlight charges it fully.
- A UV torch does it in seconds, which is why collectors carry one.
- A phone torch held close for half a minute works well enough before bed.
- Warm indoor bulbs barely charge it. That is why a watch worn indoors all evening looks dim.
Colours
Green-glowing lume is the brightest to the eye. Blue lume glows longer but looks dimmer. Cream or "vintage" coloured lume trades some brightness for an old-watch look.
Tritium
Some watches use tiny glass tubes of tritium gas, which glow all the time with no charging. Tritium has a half-life of about 12 years, so the tubes are about half as bright after 12 years. Old tritium paint on a vintage dial has stopped glowing and turned creamy, and collectors value that patina.
Radium
Watches from before about the 1960s may have radium paint, which is genuinely radioactive. It is safe to wear with the case closed. Never open the case or scrape the dial, and leave relumes or restoration to a watchmaker equipped for it.
In the dark
Switch the sync screen to dark mode and the dial's lume glows, so you can see how a lumed diver reads at night.